1. Field of the Invention
The present invention relates to X-ray generator control systems. More particularly the present invention relates to methods and apparatus for extending the life of an X-ray tube and for predicting the imminent failure of the X-ray tube by controlling the operation of the filaments in X-ray tubes having cathodes containing at least two filaments.
2. Description of Related Art
X-ray systems typically comprise an X-ray generator, an X-ray tube and a controller. The X-ray generator generates the high power input signals required by the X-ray tube to produce X-rays. The controller generally controls the operation of the X-ray system.
X-ray systems are used in a wide variety of different applications, requiring a variety of different X-ray tube configurations. Generally, the configuration used in a particular application is governed by the amount and intensity of X-rays needed for that application.
In medical applications, the X-ray apparatus must provide sufficient radiation to produce clear images of a patient's internal structures while minimizing the amount of radiation delivered to the patient. For example, radiography requires large doses of high intensity X-ray beams which are emitted toward relatively large subjects. X-radiation output for such radiography has been reduced for some applications through the use of digital radiography systems. On the other hand, fluoroscopy requires much smaller dosages of radiation but over an extended time period.
The X-ray tube is an essential element in medical and other imaging systems. Generally, an X-ray tube is comprised of an anode and a cathode enclosed in an X-ray tube housing having a tube window or port. The function of an X-ray tube is to produce and direct X-rays onto an imaging medium.
X-rays are produced when fast moving electrons contact a target surface. Electrons are formed at the cathode when a cathode filament is heated to high temperatures. The electrons are then accelerated across a large potential difference to collide with the target anode. Upon collision, the electrons interact with atoms of the target to produce X-radiation energy, which is then directed outside the tube onto an imaging medium. The amount of X-radiation emitted from the tube is dependent at least in part on the number of electrons produced and thus the temperature of the filament.
A primary concern in X-ray systems is the maximization of the life span of the X-ray tube. X-ray tubes are a relatively expensive recurring cost in the operation of an X-ray system. The high temperatures which are required cause wear on the entire X-ray tube, most significantly on the filament by vaporization of the filament atoms. Over continued use, the filament is substantially weakened and ultimately fails. Thus, the life of the filament primarily determines the life the X-ray tube.
Another concern in the operation of X-ray systems is preventing unexpected X-ray tube failures. The ultimate failure of an X-ray tube is difficult to predict. Because an X-ray system is inoperable without a functioning X-ray tube, such failure adds significant cost and inconvenience to the normal operation of an X-ray system, such as in a hospital X-ray room. This is especially true for small hospitals and clinics, where backup equipment is not available and for Emergency Rooms and during critical medical procedures, where the life of a patient sometimes depends on the reliability of the equipment.
A number of attempts have been made to reduce the stresses on X-ray tubes, thereby extending their operation. Various materials have been utilized to improve their heat absorption ability. For instance, the anode may be constructed of copper material in order to maximize the transfer of heat away from the electron target. In addition, the anode can be made to rotate, thus extending the target area and increasing heat dissipation during operation. The tube may also be filled with a coolant fluid, such as oil, which is circulated to further dissipate heat. Each of these improvements are generally aimed at protecting the anode from excessive heat exposure.
Similarly, attempts have been made to minimize filament failure. Conventional X-ray tubes typically contain two filaments of different sizes which may not be used interchangeably. Therefore, when either filament fails, the tube has to be replaced. X-ray tubes having multiple identical filaments are also available. In such tubes, after a failure of the first filament, operation of the X-ray system need only be delayed long enough to switch operation to a new filament. The problem with these X-ray tubes having multiple identical filaments is that they provide only a redundant alternative to immediate break-down of the X-ray system. Thus, after a first failed filament, an operator still has little warning for subsequent filament failures. At this point, energizations can continue, but with each additional energization the risk of unexpected system failure increases. In essence, after a first failed filament, the tube has the same vulnerability to failure as if it contained only a single filament.
Alternatively, an operator can replace a dual filament X-ray tube with dual identifcal filaments entirely in order to circumvent the risk of critical failure. In this scenario, the operational filament, after only a few energizations, is wasted. Therefore, although such a dual filament X-ray tube provides a good back-up in the event of a initial filament failure, it does not provide a reliable way to predict the ultimate failure of the X-ray tube as a result of the second or last filament failure. In addition, such a mode of operation does not improve the life of a filament beyond that of normal operation.